Diverse applications require low-viscosity fluids which can easily be applied to any desired surfaces and subsequently can be converted, through a build-up in molecular weight, to layers having a variety of properties according to application. Important applications of such systems include, in particular, coatings and adhesives. One particular form of adhesives are pressure-sensitive adhesives, which are distinguished by permanent tack.
Starting compounds used for organic surface layers are frequently low molecular mass or oligomeric systems which have been provided with at least two reactive groups, via which a build-up in molecular weight is realised in the operation of curing. Typical representatives are thermally curable epoxy resins and radiation-curable acrylic resins. The reactive groups in this case are oxiranes or unsaturated acrylate or methacrylate groups. Since, naturally, the distance between two adjacent reactive groups in the low molecular mass or oligomeric starting compounds is low, the operation of curing generally produces thermosets, in other words networks with a very high nodal density. For some applications, however, it is desirable for the mechanical properties of the surface layer under preparation to correspond not to those of thermosets but instead to those of elastomers, in other words networks with a relatively low nodal density. Such applications also include the field of pressure-sensitive adhesives. In order to achieve the build-up of networks that can be used as pressure-sensitive adhesives, it is necessary to choose starting compounds in which the reactive groups have a relatively large distance from one another, but still have a sufficiently low viscosity for good processing properties. Preferential suitability for this purpose is possessed by functionalized polymers with a low degree of polymerization.
The simplest approach, from a conceptual standpoint, for corresponding starting compounds is represented by telechelic polymers. This term refers to linear polymers which carry a functional group at both chain ends. If the chemistry of the functional groups has been chosen such that they are able to react with a crosslinker, then the free network arc length in the resulting network is defined by the degree of polymerization of the telechelic starting polymer. The literature has described a range of different kinds of telechelic polymers and their applications [O. Nuyken, S. Pask in “Encyclopedia of Polymer Science and Engineering, H. F. Mark, N. M. Bikales, C. G. Overberger, G. M. Menges (eds.), 2nd ed., 1985, Wiley, New York, Vol. 16, p. 494ff]. One important application of telechelic polymers is represented by the field of polyurethanes [G. W. Becker, D. Braun (eds.), Kunststoff Handbuch, Vol. 7, Polyurethane, 3rd ed., 1993, C. Hanser, Munich]. Here, hydroxyl-functionalized oligomers, for example, are reacted with diisocyanates, so that thermoplastic elastomers are obtained by chain extension. Within the field of the polyurethanes a restriction is generally made to those starting polymers which are obtained via a polycondensation reaction (such as polyethers and polyesters, for example) or via anionic addition polymerization (such as polydienes and hydrogenated polydienes, for example), since in those cases, for technical reasons associated with the polymerization, end functionalization is possible in an advantageous way. Polymers prepared by free-radical addition polymerization, in contrast, allow no such simple functionalization of both chain ends. If polymers of this kind are wanted as starting polymers, use may be made of controlled-growth free-radical polymerization methods where the functionalization is already present in the control reagent (see, for example, EP 237 792 of Akzo). There is, however, an expectation of high costs for such functionalized regulator molecules, and, owing to the low degree of polymerization, these costs impact directly on the costs of the functionalized polymer.
Another easy way of preparing functionalized polymers which can be realised in particular for free-radically polymerizable systems is the use of functional comonomers when building up the polymers. Although the degree of functionalization in these materials is not as well defined as in telechelic systems, such definition is also not needed in every case. It is important that the starting polymers have good coating properties, which are manifested in the melt viscosity at processing temperature. Since the melt viscosity depends significantly on the degree of polymerization, it is preferred to use starting polymers of low molar mass, and to some extent oligomers. In order to generate advantageous crosslinking behaviour and good product properties, care must be taken to ensure that the short-chain starting polymers are as far as possible fully functionalized. A maximal degree of functionalization, however, is desired, because (a) apart from the possibility of subsequent crosslinking there should be no change in any further polymer properties, particularly the dynamomechanical or rheological properties, which derive in particular from the comonomers used as majority component, and (b) complete reaction of the functional groups during a subsequent crosslinking reaction is possible, so that any latent readiness to react in the end product, such as a postcrosslinking potential, for example, of these reactive groups is ruled out, without the distance between adjacent nodes becoming too close.
A copolymer prepared by conventional free-radical addition polymerization is distinguished not only, like all synthetic polymers, by a distribution of molecular masses but also, even for the rare case of all monomers used having the same copolymerization parameters, by a distribution in composition. This means that the exact composition of the copolymers varies from chain to chain, for statistical reasons. If a copolymer is to be prepared from a monomer mixture in which one variety of monomer, a functionalized type, for example, is present only to a small extent, then individual polymer chains may be formed in which no monomers of this functionalized type are incorporated. The lower the average molar mass of the copolymer, the higher the probability that individual chains have not received any functionalization. In any subsequent crosslinking step, such chains cannot be integrated into the network that forms. If the copolymer is a material with a low softening temperature, as is the case for pressure-sensitive adhesives, then these unfunctionalized polymer chains remain in the network as migratable components. They then act as plasticizers and are potentially capable of accumulating on surfaces.
A further problem associated with copolymerization arises through a difference that may possibly occur in the copolymerization parameters of the comonomers used, these parameters depending, inter alia, on the chemical nature of the comonomers [J. Brandrup, E. H. Immergut, E. A. Grulke (eds.), Polymer Handbook, 4th ed., 1999, Wiley, New York]. Since functionalized monomers differ in this respect above all from unfunctionalized monomers, it is frequently necessary to take account of this aspect when building up functionalized copolymers. A binary monomer mixture may be cited by way of example. If the functionalized comonomer has a copolymerization parameter of significantly more than 1, and the unfunctionalized comonomer has a copolymerization parameter significantly less than 1, which is the case with the combination of 2-hydroxyethyl methacrylate as functionalized comonomer and n-butyl acrylate as unfunctionalized comonomer, then 2-hydroxyethyl methacrylate will be incorporated preferentially into the polymer chain and consumed preferentially. In chains which are not formed until towards the end of the polymerization, it is then not possible for any 2-hydroxyethyl methacrylate to be incorporated, and so these chains remain unfunctionalized. As an attempt to prepare copolymers with a narrow distribution of composition, the literature describes metering techniques [see, for example, I. N. Askill, D. K. Gilding, Polymer, 1981, 22, 342]. This technique, known as the semibatch process, must be optimized for every comonomer mixture, however, and necessitates a reactor fitted with a metering system [A. Echte, Handbuch der technischen Polymerchemie, 1993, Wiley-VCH, Weinheim].
An important basis for materials which are employed in surface layers, whether as coatings or as pressure-sensitive adhesives, are (meth)acrylate copolymers. The reason for this is that the properties of this kind of copolymers can be adjusted through the choice of the comonomers used, for which a broad range of different kinds of acrylate and methacrylate comonomers are available as starting materials. Depending on the choice and proportion of the comonomers used it is possible, for example, to control the mechanical properties or the polarity of the resulting copolymers. (Meth)acrylate copolymers are notable, furthermore, for high stability with respect to temperature, UV radiation and oxidation, so making them especially suitable for applications requiring weathering stability. A particularly appropriate functionalization possibility for (meth)acrylate copolymers is that of statistical functionalization through the use of functionalized comonomers, since they can be prepared by a free-radical polymerization process. By (meth)acrylate copolymers are meant, for the purposes of this invention, copolymers synthesized primarily from acrylate monomers and/or methacrylate monomers.
Processes for preparing short-chain (meth)acrylate copolymers as starting polymers for surface layers, known as acrylic resins or acrylate resins, are known from the literature. DE 26 35 177 of BASF AG describes acrylate resins which are polymerized from a comonomer mixture containing 10%-35% by weight of α,β-olefinically unsaturated carboxylic acids, in order to introduce functional groups, and which are polymerized without the presence of regulators at more than 160° C. The polydispersity of these acrylate resins is said to be 2.5. U.S. Pat. No. 5,082,922 of Valspar Corp. describes the derivatization of acrylic resins obtained by free-radical polymerization from a comonomer mixture containing at least 5% of an ethylenically unsaturated comonomer containing hydroxyl or carboxylic acid groups as functional groups. DE 22 40 312 of Ford Werke AG describes crosslinkable methacrylate resins which are obtained by free-radical polymerization and recovered by the use of 8% to 30% of glycidyl methacrylate as a functionalized comonomer.
For conventional free-radical polymerization the achievable polydispersity, given by the ratio of weight average to number average of the molecular weight distribution of the polymer produced, is ideally 1.5 for termination exclusively by disproportionation and ideally 2.0 for termination exclusively by combination. As a result of chain transfer events, however, the distribution of polymers obtained by conventional free-radical polymerization tends to be relatively broad. This distribution of molar masses results in a distribution not only in the area of the high molecular mass fraction but also in the area of the low molecular mass fraction. The formation of a high fraction of low molecular mass components during the polymerization may prove, in accordance with what has been said above, and solely on statistical grounds, to be deleterious for the incorporation of those monomers present in a low proportion in the monomer mixture.
JP 10226714 A describes copolymers which contain (meth)acrylates and have a narrow monomodal molar mass distribution. The copolymers disclosed are prepared by means of anionic polymerization. For this reason it is necessary to convert reactive and in particular acid-containing comonomers into less reactive groups prior to the polymerization. Where the comonomer mixture includes an acid-containing component, successful anionic polymerization is not possible. Acid-containing copolymers are obtained by using silyl-protected comonomers, which when the polymerization is over have to be deprotected, in an additional step. The copolymers proposed are intended as resistant material for ArF excimer lasers.
DE 43 24 801 discloses (meth)acrylate copolymers which contain OH groups and have weight-average molar masses of below 8600 g/mol. The copolymers described are prepared by free-radical polymerization and are said to have a narrow monomodal distribution. The (meth)acrylate copolymers are intended for coatings applications.
DE 103 06 431 likewise describes (meth)acrylate copolymers with a monomodal distribution. The (meth)acrylate copolymers are prepared by means of free-radical polymerization and are obtained as particles dispersed in an emulsion. The particles are intended for coatings applications.
None of these patent specifications describes polymers which at low molar masses have a small difference, relative thereto, between the peak molar mass and the smallest molar mass determined at a given point. Furthermore, none of these patent specifications describes polymers with a narrow molar mass distribution which are to be used as adhesives and are therefore subjected to crosslinking.
Adhesive compositions based on poly(meth)acrylates are known from publications including DE 100 30 217 and DE 102 12 899. The polymers are crosslinked for example by means of actinic radiation. The pressure-sensitive adhesives described in DE 100 30 217 have a weight-average molar mass of at least 250 000 g/mol. None of these pressure-sensitive adhesives comprises polymers which have, at low molar masses, a small difference, relative thereto, between the peak molar mass and the smallest molar mass determined at a given point.
DE 42 20 807 discloses a film-forming binder based on vinyl polymers having a narrow molecular weight and a broad molecular weight distribution. DE 41 27 513 discloses polyacrylate resins suitable as binders. The polyacrylate resin may be a hydroxy-functionalized polyacrylate having a bimodal distribution. No polymers are described that suggest any indication of the advantageous molar mass distributions of the invention.
It is an object of the invention to eliminate the disadvantages of the prior art. The intention in particular is to specify a functionalized polymer or a pressure-sensitive adhesive which can be produced on the basis of functionalized copolymers with a low molecular weight and a reduced fraction of non-functionalized chains, via a free-radical polymerization process, preferably in batchwise operation.